There have conventionally been proposed hydraulic control systems, each of which is mounted on a hydraulic excavator and has a main hydraulic pump and a boom cylinder and arm cylinder as a first hydraulic cylinder and second hydraulic cylinder driven by pressure oil delivered from the main hydraulic pump. This conventional art is provided with a directional control valve for a boom as a first directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the boom cylinder, a directional control valve for an arm as a second directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the arm cylinder, a boom control device as a first control device for selectively controlling the directional control valve for the boom, and an arm control device as a second control device for performing a switching control of the directional control valve for the arm, and is also provided with a communication control means for communicating a rod chamber of the boom cylinder and a bottom chamber of the arm cylinder with each other when a bottom pressure of the arm cylinder has increased to a high pressure equal to or higher than a predetermined pressure. (see, for example, JP-A-2002-339907).